4 customer connection district heating training … customer connection district heating training...

4 Customer Connection

District Heating Training Course

B09HV_en

16.04.2002

Mikkeli Polytechnic Siemens Building Technologies

Building Automation

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Siemens Building Technologies District Heating Training Course B09HV_en

Building Automation 16.04.2002

Siemens Building Technologies Ltd.

Building Automation

Gubelstrasse 22

CH-6301 Zug

Tel. +41 41-724 24 24

Fax +41 41-724 35 22

www.landisstaefa.com

Mikkeli Polytechnic © 2001 Siemens Building Technologies Ltd.

Subject to change

http://www.landisstaefa.com/

3/19


Building Automation Table of contents 16.04.2002

Table of contents

4.1 Substation ..................................................................................................... 5

4.1.1 Prefabricated substation ............................................................................... 5

4.2 Connection schemes .................................................................................... 6

4.3 Substation equipment ................................................................................... 7

4.3.1 Heat exchangers ........................................................................................... 7

4.3.2 Pumps ........................................................................................................... 9

4.3.3 Control system ............................................................................................ 10

4.3.4 Other equipment ......................................................................................... 10

4.4 Space heating ............................................................................................. 12

4.4.1 Radiator network ......................................................................................... 12

4.4.2 Floor heating ............................................................................................... 15

4.5 Other systems ............................................................................................. 16

4.5.1 Connection schemes .................................................................................. 16

4.5.1.1 Direct or indirect .......................................................................................... 16

4.5.1.2 Open or closed............................................................................................ 16

4.5.1.3 4 pipe system .............................................................................................. 17

4.5.1.4 Substations for group of buildings............................................................... 17

4.5.2 Trends ......................................................................................................... 17

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Building Automation Customer connection 16.04.2002

4 Customer connection

Training is familiarised trainee with the customer connection of DH. The trainee will be

able to:

- understand different connections of substations

- name the main components of modern substation

- understand the benefits of prefabricated substation

- name the different types of heat exchangers and other equipment

- explain the design and control strategies of SH systems for DH

The customer connection consists of the metering centre and substation. The metering

and metering centre is discussed more in chapter 8.

The target in DH-system is to make and to distribute heat energy over a large area as

economically as possible. System must be economical both to the customer and the

heating company (Heat Vendor). This means that high quality equipment are needed

though the whole field of DH-system.

The control of heat production plants shortly means the regulation of DH supply water

temperature and the pressure levels in DH network. The economy and the control

strategy of DH network depend also on the types of connections of the customer. In the

history of DH technology there has been a lot of different kinds of “connections”. Some

of these are presented in chapter 4.5.

After a long experience in Scandinavia the best strategy to control the economy of DH

network is to use the customer owned substations with heat exchangers. In this kind of

controlling concept customer is connected individually and customers heating networks

and DH distribution network are completely separated hydraulically by heat

exchangers. Connection of the customer oriented control and automation system

enable independent energy consumption for the each customer. Using the substation is

also important because the pressure and the temperature conditions are varied a lot in

DH distribution network. Other important point is to protect buildings for water damages

if secondary network is leaking.

Fig 4- 1 Prefabricated substation

Learning goals

Why a substation

5/19



4.1 Substation

In a substation there are heat exchangers, control system and other equipment needed

for heat delivery to space heating, ventilation, domestic hot water and other purposes.

Every independent heating circuit has own heat exchanger and control system.

What do we need to choose a substation correctly? At first it is important to contact the

district heating company to get information about the design background like

dimensioning temperatures and the normal and variation of pressure-difference

conditions in DH network. At second, every building is different and heat demand of

every heating circuit must be calculated according a local authority requirements. The

best result is achieved by choosing an own heat exchanger and other needed

equipment for each heating circuit individually.

The calculated heat demand and the dimensioning temperatures lead on to the needed

flow in primary and in secondary side. Selection of the heat exchangers and other

substation equipment depend on flow capacity and permissible pressure losses of the

whole substation. In Finland maximum permissible pressure losses (pressure drop) is

20 kPa in the DH primary side.

4.1.1 Prefabricated substation

Prefabricated substations consist of the finished installation of all mechanical and

electrical equipment shown in technical documentation (connection scheme). This

means that

- thermal insulated heat exchangers

- control system of the substation

- alarms

- pumps

- sensors

- other equipment (strainers, thermometers and differential pressure control valve)

are connected electrically and pipes and stop valves are thermal insulated. All

equipment, joints and flow directions are marked with texts and arrows according the

HVAC-plans.

If the substation unit must be connected to centralised monitoring (building automation

system, Direct Digital Control DDC), the control equipment can be supplied with the

substation strictly in accordance with the desired itemised list. If there are valid reasons

for excluding the control units from the scope of supply, the control valves, their

actuators and temperature sensors should, however, be included. The field equipment

is wired to a terminal box, resulting in a complete substation unit.

Design principles

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Fig 4- 2 Connection scheme

4.2 Connection schemes _____________________________________________________________________

It is important to present the substation connection and included components in one

scheme with dimensioning such as

- heat demands

- temperatures

- flow capacities

- pressure losses

- DN and kvs-values of control valves.

In accordance with the scheme it is possible to ask an offer from DH substation

manufactures and compare them each other. Customer can named the manufacture of

equipment as control system and pumps to use in substation, they can be chosen

freely.

Detailed connection schemes are presented in chapter 6.

All the space heating functional temperatures in secondary side depend on the outdoor

temperature and the functions of all equipment of the substation must be explained in

the connection scheme.

How to connect heat exchangers to DH network and secondary network depends on

the heat demands and permissible pressure losses in primary and secondary side. The

aim of different connections should be better cooling of the DH water; this is achieved

by using counter flow connection with heat exchangers. Better cooling reduces

pumping energy and thermal losses of the DH network. In CHP production better

cooling increases the efficiency of electricity production also.

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Fig 4- 3 Instruction for presenting connection schemes and dimensioning tables

4.3 Substation equipment _________________________________________________________________________________

All components inside the boundaries of the substation are called as substation

equipment. Boundary means the delivery limit of the prefabricated substation shown in

connection scheme.

4.3.1 Heat exchangers

Modern substation unit for connecting building to district heating networks is fitted with

brazed plate heat exchanges. The heat exchange surfaces are made from stainless or

acid-resistant steel. This type of heat exchanger has some advantages like reliability,

lightweight and large output for its size. As a disadvantage of the brazed heat

exchanger is really small inside water volume, which may cause problems with

regulating the DHW temperature.

Fig 4- 4 Brazed plate heat exchanger principle

Brazed heat exchanger

Connection scheme

Temperatures of the networks and

explanations of their function

DIMENSIONING

TABLE 2

Dimensioning

of substation

DIMENSIONING

TABLE 1

Technical data of

heating system

Further information from heating

plant or from contactor

Title

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Different kind of heat exchangers is available for district heating use. If needed to use

plate heat exchangers with gaskets, manufacture should guarantee the durability and

elasticity of gaskets and other elastic parts in DH conditions. In normal DH-water there

is no impurities, why the heat exchanger must be opened for cleaning purposes.

Heat exchangers made from copper coil and surrounded by steel shell, are critical for

velocity of the water flow. In a copper pipe critical velocity of water flow is 1,0 m/s.

Higher velocity can caused corrosion damages in a short period. Copper pipe is quit

critical for water quality like impurities and pH-value of water.

Fig 4- 5 Shell and coil heat exchanger

1 Primary supply

2 Secondary supply

Earlier shell and tube exchangers were used a lot in DH systems. These heat

exchangers needed physically a large floor area and the cooling efficiency of the DH

water was lower than other type of heat exchangers, specially compared to plate-type

exchangers.

Fig 4- 6 Shell and tube heat exchanger

1 Primary supply

2 Secondary supply

Heat exchanger with

gaskets

Shell and coil heat

exchanger

Shell and tube heat

exchanger

2

1

1

2

9/19



Regardless of the type and the material of the heat exchanger, the basic idea is that

heat exchangers shall be dimensioned and build so that cooling of the DH water is as

effective as possible in all conditions and all water flow run along the heat exchange

surfaces. If the temperature difference in the secondary network is small (e.g. in floor

heating systems), a part of the secondary flow can pass the heat exchanger. It is

forbidden to mix the secondary supply water with the secondary return water. Heat

exchangers must have thermal insulation and all joints must be clearly marked.

4.3.2 Pumps

Pumps are needed to circulate the secondary water for heating and for domestic hot

water re circulation use. Only centrifugal pumps are used in DH substations. For

dimensioning the pump capacity (flow) and pressure loss in piping and in heat

exchanger (head) are needed. It is recommended to use pumps, which rotational speed

is not over 1500 r/min. Higher rotation speed may caused problems with the

permissible noise level in buildings.

Selection of pumps must be able to fulfil the maximum required flow and pressure

(pump head). To ensure optimum operation economy, the pump should be selected so,

that duty point location is on the highest part of the efficiency curve.

Pump markings like

- manufacture

- type

- capacity

- head

- electrical power demand

must be clearly and durability marked on the pump. In some cases it is recommended

to have a stand-by pump for maintenance meaning for critical secondary network.

Fig 4- 7 Pump head / volume flow

1 characteristic curve of pump

2 piping system pressure curve

Selection

kPa

dm / s3

12

10/19



4.3.3 Control system

The DH system based on customer oriented control is economical in many ways. The

DH company doesn’t have to make control system investments for every customer and

it can concentrate the quality (temperature and pressure level) of DH supply water. For

customer’s perspective investments on control system are leading to financial benefits

in quite short run. By a control system customer has a straight influence to the living

conditions like indoor temperature. More likely is that customer’s interest concentrates

on expenses of the heating and the control system is the tool to reduce these

expenses. This arrangement presumes that control systems is planned and build for

DH network conditions and considering special needs of the customer.

Control systems have been used in developed DH systems from the beginning.

Traditionally control system has consisted at least controllers for heating and domestic

hot water supply. This kind of system is part of the substation even today. It guarantees

adequate adjustment possibilities for building heating.

Technical development has made substation as a part of the building automation. This

automation enabled to monitor different building operations, but requirements for adjust

temperatures in buildings are the same than in control system.

The basic functional requirements for the control or automation system connected in

DH system are identical. Main task is taking care of the

- space heating temperature

- ventilation temperature

- DHW temperature

in the building. In space heating it means that dynamic heating properties of the

building and the external energy coming e.g. from the sun shall utilised by the control

system. In DHW it means that supply water temperature is constant.

Functional requirements for the control system in normal DH network conditions are as

follow. The control system is

- designed

- chosen

- dimensioned

- installed

- adjusted

so that greatest constant deviation from the adjusted water temperature value is 2 C.

Greatest momentary deviation from the adjusted temperature value in DHW systems is

10 C, in space heating systems 5 C and in ventilation systems 10 C. Accepted

amplitude of oscillation in DHW systems is 2 C and in other systems 0,5 C.

4.3.4 Other equipment

In substations only ball valves or butterfly valves are used for stopping water flow.

Joining method is welding or flanged joint. Recommended materials in closing surfaces

of valves are stainless steel and acid-proof steel.

Control

Automation

Requirements

Shut-off valves

11/19



Fig 4- 8 Steel ball valve

Substation must be equipped with safety valve for water expansion and in case of

breakdown of the control system. The best place to install secondary side expansion

pipe is in the suction side of the pump connected to the return pipe.

Pipes used in substation must sustain in DH temperature and pressure conditions

unchanged. The most common materials used in piping are steel and copper. Joining

methods are welding or brazing.

It is recommended to use strainers with plate-type heat exchangers both, in primary

and in secondary side, assure uninterrupted usage of the substation.

Thermometer and other sensor pockets are made of stainless steel, acid-proof steel of

copper.

All materials used in substation manufacturing will be chosen so that they sustain in

normal DH network conditions the whole living time of the substation. It is forbidden to

use cast iron as a primary side material.

If the pressure difference of the DH network fluctuates more than 400 kPa (4 bar),

recommended to use the pressure difference valve (regulation).

High pressure difference caused problems for the regulation result and may caused

cavitation in control valves.

Combination valve is a control valve with pressure difference control function.

Fig 4- 9 Combination valve

More detailed information is in chapter 7.

Safety valve

Pipes

Strainers

Sensor pockets

Materials

Pressure control

12/19



4.4 Space heating __________________________________________________________________________________

SH systems of the building connected to DH system should be designed and installed

so, that

energy consumption (kWh) and heating capacity (kW) is as low as possible

control system can utilise internal heat sources (sun, lights, occupants,

machines etc.)

heat capacity of the structures can be utilised for saving energy and thermal

power

temperatures of SH networks are adjustable according the outside temperature

and at the lowest possible level

control system can operate in variable DH pressure conditions

SH networks need as little hydraulic balancing as possible.

All the circuits of SH networks should be set in hydraulic balance. This is done by using

balancing valves both for each network line and for each radiator. By these balancing

valves you set beforehand the needed water flow for each line and radiator.

The expansion system of SH networks should be so called closed system. In an open

system there is always great possibility to get air (O2) to your system and the danger for

corrosion in your network is obvious. In normal buildings the membrane expansion

vessels are used. In high rise buildings you can use compressor system or pump

controlled system. The expansion vessels should be dimensioned according to physical

properties of heating fluid. For the normal heating water the expansion is approximately

2,5 … 3,5 % of the total water volume of network. Note that the water temperature also

in secondary side of DH could rise up to 110 … 115 oC if for instance the control valve

is jammed in open position.

The safety valves should be dimensioned according to local pressure regulations and

standards.

4.4.1 Radiator network

In DH connected radiator network it is possible to use all normal network structures.

General aims

Hydraulic balancing

Expansion and safety

Systems

1

2

3

13/19



Fig 4-8 Structures on radiator networks

1 Normal 2-pipe system

2 2-pipe system with reverse return (Tichelman’s system)

3 1-pipe system

In normal 2-pipe system (Fig 4-8) the radiators are connected parallel to pipelines. In

this system the advantage is easy dimensioning of radiators because each radiator has

the same dimensioning temperature (logarithmic mean temperature between the

radiator surface and indoor air temperature). The greatest disadvantage is the natural

hydraulic unbalance of this system. Each radiator has different pressure losses so the

normal 2-pipe system needs quit a lot work in hydraulic balancing.

In 2-pipe system with reverse return (Tichelman’s system; Fig 4-8) this disadvantage is

avoided. Now each radiator has about the same pressure losses so the hydraulic

balancing is quite easy.

In 1-pipe system (Fig 4-8) the radiators are connected series to pipelines. In this

system the disadvantage is laborious dimensioning of radiator because each radiator

has different dimensioning temperature (logarithmic mean temperature between the

radiator surface and indoor air temperature).

In following picture is presented the most common normal 2-pipe system for multifloor

building.

Fig 4-9 Normal 2-pipe system for multifloor building.

1 Control valve

2 Balancing valve for lines

3 Thermostatic radiator valve with balancing device

4 Expansion vessel

5 Safety valve

5

4

1

2

3

DH

14/19



The normal design temperatures for new DH connected radiator network are

supply: ts = 70 oC (=maximum temperature in new buildings)

return: tr = 40 oC (=maximum temperature in new buildings)

The return temperature of DH connected SH network can be lower than in most non-

condensing boiler heated networks because there is no risk for corrosion. Of course

this is up to the fuel you use.

Note the influence of temperature difference T = ts - tr for the characteristic of radiator

and further for thermostatic radiator valve characteristic (Fig 4-10).

The design temperatures of radiator network shall be discussed more detailed in

chapter 6.

According to above named general aims the best control strategy of radiator networks

is to regulate the supply water temperature according to outdoor temperature. Because

this is centralised control for whole network you need an extra system to control the

indoor air temperature in each room.

Control system for radiator network shall be discussed more detailed in chapter 6.

Normally you use thermostatic radiator valves for controlling indoor temperatures in

each room of the building. Another important task for the thermostatic radiator valve is

to utilise the internal heat sources and save the heating energy.

In figure 4-10 is presented the heating capacity of typical radiator as a function of water

flow. The parameter is temperature difference T = ts - tr.

Fig 4-10 Emitting heating capacity of typical radiator as a function of water flow. The

parameter is temperature difference T = ts - tr.

In the figure 4-10 you can see that the heating capacity is much more non-linear when

the temperature difference T decrease. So if T = 30 K there is less requirements for

the thermostatic radiator valve to get linear response between the emitted heating

Temperatures

Control system

Thermostatic radiator valve

%

%q/q

100

100

T

15/19



capacity and flow. If T = 10 K the curve is quite non-liner upwards so you need the

valve which have characteristic strongly downwards to get the linear response.

Note also that the radiators might be difficult to control at small loads because the

heating capacity is very dependant on the flow. In real rooms the thermostatic radiator

valves work most of the time at quite small loads.

4.4.2 Floor heating

Due to the local thermal discomfort caused by the surface temperatures of floor the

water temperatures of floor heating are restricted. In Table 4-1 is presented the surface

temperatures of floor according to EU thermal comfort recommendations.

Table 4-1 The surface temperatures of floor in different thermal comfort

categories (CR1752:1998. Design criteria for indoor environment)

Category Percentage of

dissatisfied

due to warm floor

PPD

Surface temperature

of floor

% oC

A < 10 19 – 29

B < 10 19 – 29

C < 15 17 – 31

On the Table 4-1 you can see that the highest floor temperatures can be in range of 29 oC – 31

oC. This means that the highest supply temperature of floor heating can be only

about 40 oC – 45

oC. It is essentially lower than in normal radiator network.

The normal design temperatures for new DH connected floor heating network are

supply: ts = 40 oC

return: tr = 30 oC

Because the design water temperatures for floor heating and radiator network are

different you can not connect the radiators and floor heating loops to the same network.

You must have own network for radiators and own for floor heating loops. It is possible

to heat up these two networks by one DH heat exchanger. If so you need own mixing

group with 3-way control valve for floor heating network. Nowadays it is preferable and

even more economical to use own heat exchanger for radiator network and own for

floor heating network. This is discussed more detailed in chapter 6.

As for radiator network also for floor heating networks the best strategy is to regulate

the supply water temperature according to outdoor temperature. Because this is

centralised control for whole network you need an extra system to control the indoor air

temperature in each room. In floor heating you can use control system for each room or

for the group of rooms.

Control system for floor heating shall be discussed more detailed in chapter 6 and 7.

Temperatures

Control system

16/19



4.5 Other systems

In this chapter some features of present Central and Eastern Europe systems will be

described. It is essential to understand some basics of those systems to find out

possible renovation solutions and possibilities.

4.5.1 Connection schemes

Heating systems can be either

- Open or closed

- Direct or indirect

The differences between these systems are described in following.

4.5.1.1 Direct or indirect

Indirect

The term ‘indirect’ has been used to describe an indirect space heating system, which

means that the district heating network and the radiator circuit are hydraulically

separated from each other by means of heat exchangers (see also “direct”). In addition,

an equivalent term 'independent' is used.

Direct

The term ‘direct’ has been used to describe a direct space heating system, which

means that district heating water circulates also in the radiator circuit. In addition, an

equivalent term 'dependent' has been used.

Fig 4- 11 Direct connection of space heating

4.5.1.2 Open or closed

Closed system

The term ‘closed’ has been used to describe the closed DHW system, which means

that domestic hot water has been produced (i.e. heated) by heat exchangers either

situated in the heat exchanger station (so called group heat exchanger) or in the

buildings

Open system

The term ‘open’ has been used to describe the open DHW system, which means that

domestic hot water is the same as district heating water. It is the opposite to the closed

DHW system.

1)

1) Ejector for mixing return water into supply water

17/19



Fig 4- 12 Open and direct DH connection

4.5.1.3 4 pipe system

The term ‘4-pipe system’ has usually been used to describe a variation of the closed

DHW system, which means that domestic hot water has been produced (i.e. heated) by

heat exchangers situated in the heat exchanger station (so called group heat

exchanger) and led to the building with one DHW supply pipeline and a re-circulating

return pipeline (the re-circulating pipeline quite usually is missing in Eastern Europe

systems).

4.5.1.4 Substations for group of buildings

Building level substation is normal in Scandinavian DH systems. In Eastern and Central

Europe DHW is quite often produced in large group heat exchangers.

In these kind of open systems the demand of supply water into the system is huge

during the peak DHW consumption period.

Space heating can be produced in heat exchangers or the connection can also be

direct.

4.5.2 Trends

In Eastern and Central Europe more and more DH systems will be changing towards to

Western system presented in this paper. There will be different kinds of problems

dimensioning and planning the systems during this transition period, when both

systems are in operation.

Trend in DH systems are:

- Lower temperature levels in distribution and consumption

- Plastic pipes will come more popular

- Amount of small CHP plants will increase

T1)

T1)

1)

1) Ejector for mixing return water into supply water

2) Space heating

3) DHW

2)

18/19


Building Automation Index 16.04.2002

Index

19/19


Building Automation 16.04.2002

Siemens Building Technologies Ltd.

Building Automation

Gubelstrasse 22

CH-6301 Zug

Tel. +41 41-724 24 24

Fax +41 41-724 35 22

www.landisstaefa.com

© 2001 Siemens Building Technologies Ltd.

Subject to change

http://www.landisstaefa.com/

4 customer connection district heating training … customer connection district heating training...

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